Preheat oven to 350°F. Combine oats and pecans (if desired) with a sprinkle of cinnamon. Spread out on a baking sheet in a thin layer and bake for 4-6 minutes until golden and toasted. Remove and set aside. Reduce oven temp to 325°F.
Rinse and core the apples, but make sure not to cut through the bottom. You can use an apple corer or a paring knife. Make sure seeds are removed as well (scoop out with spoon)
In a bowl, combine oat-pecan mixture with brown sugar, yogurt, melted butter and cinnamon. Spoon mixture into the cavity of each apple. Place apples in an 8x8" baking dish. Pour water in the bottom of dish.
Bake at 325°F for 30-40 minutes until tender (not mushy). Top with whipped cream or ice cream and serve.